Reduced maintenance nickel-cadmium storage cell

ABSTRACT

A reduced maintenance, vented nickel-cadmium storage cell comprising a set of positive electrodes, a set of negative electrodes in which the active mass is consolidated by a polymer, and separators constituted by at least one felt made of a substance selected from a polyamide, polypropylene, polyethylene, individually or mixed together, and without an additional membrane. The inter-electrode distance is in the range 0.2 mm and 0.5 mm, the internal pressure is maintained in the range 0 bars to 0.7 bars, and the cell contains free electrolyte in such a manner as to present a charging characteristic when charging at constant voltage in the temperature range -30° C. to +60° C. that enables the end of charging to be detected because of the existence of a sudden rise in voltage.

The present invention relates to a reduced maintenance nickel-cadmiumstorage cell intended, in particular, for applications requiring highpowers for short durations, e.g. in aviation or in back-up powersupplies for certain types of equipment.

A first so-called "vented" type of nickel-cadmium storage cell is knownfor these applications, comprising sintered negative electrodes andpositive electrodes with the gap between two successive electrodes beingin the range 0.15 mm to 0.4 mm. This gap is filled with a separatorcomprising felts associated with a cellophane membrane made of graftedpolyethylene or of microporous polypropylene. The electrode assembly isenclosed in a case of plastic material containing a certain quantity offree electrolyte. The operating pressure lies in the range of 0.2 barsto 0.7 bars relative pressure, said pressure being controlled by avalve. In its application to aviation, each storage cell, whendischarged in 15 seconds at 25° C., presents a power density of 400watts per kilogram (W/kg) to 700 W/kg.

Most remarkably, such a storage cell presents the following operatingcharacteristic: when charged at constant voltage and limited current,its voltage increases very suddenly at the moment it becomesovercharged. This increase makes it possible for storage cell chargingto be self-limiting. This operating characteristic occurs for constantvoltage charging regardless of temperature in the range -20° C. to +60°C.

In addition, the storage cell is very reliable throughout its lifetimewhich may lie in the range 5000 hours (h) to 50,000 h of operation,depending on conditions of utilization. However, it suffers from thedrawback of consuming a quantity of water proportional to the overchargeapplied thereto. This water consumption must be compensated byperiodically adding distilled water, which operation takes the batteryout of service and requires human intervention. Water consumption isabout 1 cm³ per 3 Ah of overcharge.

A second so-called "vented" type of storage battery is known havingsintered positive electrodes and negative electrodes whose active massis consolidated by a polymer. The negative electrodes are described, inparticular, in French patent number FR-A-2 586 407. The gap between twosuccessive electrodes is greater than 0.4 mm and may be as much as 1.5mm, depending on the application and the desired power density. Powerdensity may lie in the range 50 W/kg to 200 W/kg. The separator may beconstituted either by felts associated with a membrane, as in a cell ofthe first type, or else by a spacer of plastic material, in particularfor inter-electrode gaps in the range 0.8 mm to 1.5 mm. This storagecell does not operate under pressure. It has the same advantage as thefirst type of cell from the point of view charging, being self-limitingover a vast temperature range, and the same drawback from the point ofview of water consumption. In addition, it does not meet the powercharacteristics required for aviation applications.

Sealed nickel-cadmium storage cells are also known having substantiallyzero water consumption throughout their lifetime. One such cellcomprises sintered positive electrodes and negative electrodesconsolidated by a polymer, the electrodes being separated by a gap inthe range 0.1 mm to 0.2 mm. The separator is a felt of polyamide,polypropylene, polyethylene, or a mixture of these substances. Theassembly is installed in a metal container provided with a valve, andoperating pressure is usually 3 bars to 4 bars relative to pressure.

Although sealed nickel-cadmium storage cells do not suffer from thedrawback of consuming water, as mentioned above, they do not have thevery advantageous charging characteristic enabling charging to beself-limiting. Their utilization in the applications outlined aboverequires sophisticated charging systems in which charge monitoring isessential in order to verify that the pressure is not exceeded and thatno electrolyte is lost: any such loss would quickly lead to cellfailure.

The object of the present invention is to provide a nickel-cadmiumstorage cell which retains the advantages of nickel-cadmium cells of thefirst vented type, while requiring water to be added only at a reducedfrequency.

The present invention provides a reduced maintenance nickel-cadmiumstorage cell including a set of positive electrodes and a set ofnegative electrodes in which the active mass is consolidated by apolymer or is electro-deposited, the cell being characterized by thefacts that:

it includes separators constituted by at least one felt made of amaterial selected from a polyamide, polypropylene, polyethylene,individually or mixed together, and without an additional membrane;

the inter-electrode distance is in the range 0.2 mm and 0.5 mm;

the internal pressure is maintained in the range 0 bars to 0.7 barsrelative pressure; and

said accumulator contains free electrolyte above the electrodes;

thereby presenting a charging characteristic when charging at constantvoltage, at limited current or at constant current and in thetemperature range of -30° C. to +60° C. which makes it possible todetect the end of charging because of the existence of a sudden increasein voltage.

Advantageously, the active mass of the electrodes which are essentiallyconstituted by a mixture of cadmium and cadmium oxide together with abinder contains less than 5% metallic nickel, and preferably 0%.

The inter-electrode distance is preferably in the range 0.2 mm to 0.3mm.

In a preferred embodiment, said separator includes felts made ofpolyamide and polypropylene fibers having a diameter lying in the range4 μm to 20 μm, and the felt weights about 70 grams per square meter(g/m²) to 150 g/m². Felt weight is preferably in the range 90 g/m² to120 m/g².

In such a storage cell, water consumption is divided by a factor in therange 2 to 20, depending on the geometry of the cell and on chargingconditions.

Other characteristics and advantages of the present invention appearfrom the following description of embodiments given by way ofnon-limiting illustration.

In the accompanying drawings:

FIG. 1 is a graph showing voltage curves V as a function of time t bothfor prior art cells and for a cell of the invention during charging atthe same constant voltage.

FIG. 2 is a graph showing the water consumption of the FIG. 1 cellsduring overcharging.

FIG. 3 is a graph showing voltage curves V as a function of time t bothfor prior art cells and for a variant cell of the invention duringcharging at the same constant voltage.

FIG. 4 is a graph showing the water consumption of the FIG. 3 cellsduring overcharging.

Three prior art storage cells A, B, and C are compared with a storagecell D₁ of the invention.

Prior art cell A is a vented nickel-cadmium cell for aviationapplications, having sintered positive electrodes and sintered negativeelectrodes. The gap between two successive electrodes is 0.25 mm. Theseparator is formed by belts associated with a membrane of graftedpolyethylene. The operating pressure is 0.4 bars relative pressure.

FIG. 1 shows voltage curve A in volts V, as a function of time t inhours, for said cell A during charging at a constant charging current of0.1 Cn (nominal capacity) at 20° C. The sudden increase in cell voltageafter 9 hours of charging should be noted.

In FIG. 2, curve E shows the water consumption c as a percentage thetheoretical consumption for the cell A as a function of overchargingcurrent i expressed as a fraction of Cn.

Prior art storage cell B is a vented nickel-cadmium cell for industrialapplications other than in aviation. It includes sintered positiveelectrodes and negative electrodes which are consolidated by a polymer.The gap between two electrodes is 0.50 mm. The separator is constitutedby felts associated with a membrane as in cell A.

Curve B in FIG. 1 shows the variation in the voltage V of cell B as afunction of time when it is charged under the same conditions as cell A.A sudden increase in the voltage of the cell B is again observed after 9hours of charging at constant voltage. Cell B consumes water duringovercharging in the same way as cell A, as shown by curve E in FIG. 2.

Prior art storage cell C is a sealed nickel-cadmium cell. Its positiveelectrodes are sintered and its negative electrodes are polymerconsolidated. The gap between two electrodes, equal to 0.20 mm, isfilled with polypropylene felt. The electrolyte is limited and theoperating pressure is 4 bars relative pressure.

Curve C in FIG. 1 shows the voltage curve for cell C when charged underthe same conditions as cells A and B. Unlike cells A and B, it is clearthat this voltage has no sudden increase at the end of charging. Howeverwater consumption is practically nil.

Storage cell D₁ of the invention comprises positive and negativeelectrodes of the same type as those in cell B. However, its negativemass which is essentially constituted by a mixture of cadmium andcadmium oxide with a binder, is free from metallic nickel. Theinter-electrode distance is 0.3 mm. The separator is constituted solelyby felts made of polyamide and polypropylene fibers having a diameter of4 μm to 20 μm. The separator weights about 120 grams per square meter(g/m²). The electrolyte is free above the electrodes. Operating pressureis 0.4 bars relative pressure.

As for cells A and B, curve D₁ in FIG. 1 shows a sudden increase involtage after 9 hours of charging when cell D₁ is charged under the sameconditions as before. However, its water consumption, as illustrated bycurve F₁ in FIG. 2, is considerably lower than that of the two precedingcells.

Since curve D₁ shows that it is possible for charging to beself-limiting, the overcharging current i (see FIG. 2) can be low andwater consumption can be reduced by a factor of up to 10.

In addition to this advantage, cell D₁ retains the possibility of beingcharged at constant voltage at temperatures lying in the range -30° C.to +60° C., and for periods of as long as 24 hours without any danger ofthermal runaway.

It has identical discharge characteristics to a prior vented storagecell having the same geometry, and it also has the same lifetime.

FIG. 3 shows prior art curves A, B, and C analogous to those shown inFIG. 1, together with a curve D₂ relating to a variant storage cell ofthe invention.

This storage cell D₂ comprises positive and negative electrodes of thesame nature as those of cell B. However, the negative active mass whichis essentially constituted by a mixture of cadmium and cadmium oxidewith a binder is free from metallic nickel. The inter-electrode distanceis 0.3 mm. The separator is constituted solely by felts made ofpolyamide or polypropylene fibers having a majority of fibers with adiameter of 5 μm. The weight of this separator is about 100 g/m². Theelectrolyte is free. Operating pressure is 0.4 bars relative pressure.

As for cells A and B, curve D₂ in FIG. 3 shows a sudden increase involtage after 9 hours of charging, followed by a progessive reduction,when the storage cell D₂ is charged under the same conditions as above.However, its water consumption, shown by curve F₂ in FIG. 4, is veryconsiderably lower than that of the two preceding storage cells. Thisprovides a lifetime of 3 to 5 years during which electrolyte levels donot need to be readjusted of 3 to 5 years, e.g. in aircraft batteries.

Curve D₂ shows that the end of charging is easily detected by measuringthe rise in cell voltage.

In addition to this advantage, cell D₂ retains the possibility of beingcharged at temperatures lying in the range -30° C. to +60° C.

Cell D₂ has discharge characteristics which are identical to those of aprior art open cell having the same geometry, and it has the sameoverall lifetime.

In another example, the inter-electrode distance is equal to 0.5 mm andthe weight of the separator felt is 150 g/m². The chargingcharacteristic of the resulting storage cell is analogous to that ofcell D₂.

In addition, the positive electrodes of a cell of the invention may befiber or foam support electrodes or else pocket electrodes.

Batteries of storage cells of the invention having capacities in therange 5 Ah to 1000 Ah are suitable for starting applications requiringhigh powers, and for equipment back-up applications requiring peakcurrents for short periods. They may also be used for starting emergencypower generators for telephone exchanges, et.

We claim:
 1. In a reduced maintenance, vented nickel-cadmium storagecell including a set of positive electrodes and a set of negativeelectrodes in which the active mass is consolidated by a polymer or iselectro-deposited, the improvement wherein:said cell includes separatorconstituted by at least one felt made of a material selected from apolyamide, polypropylene, polyethylene, individually or mixed together,and without an additional membrane; the inter-electrode distance is inthe range 0.2 mm and 0.5 mm; the internal pressure is maintained in therange 0 bars to 0.7 bars relative pressure; and said accumulatorcontains free electrolyte above the electrodes; thereby presenting acharging characteristic when charging a constant voltage, at limitedcurrent or at constant current and in the temperature range of -30° C.to +60° C. which makes it possible to detect the end of charging becauseof the existence of a sudden increase in voltage.
 2. A nickel-cadmiumcell according to claim 1, wherein that the inter-electrode distance isin the range 0.2 mm to 0.3 mm.
 3. A nickel-cadmium cell according toclaim 1, wherein that said separator includes polyamide andpolypropylene fibers having a diameter lying in the range 4 μm to 20 μm,with the weight of the separator lying in the range 70 g/cm² to 150g/cm².
 4. A nickel-cadmium cell according to claim 1, wherein that theactive mass of the negative electrodes contain less than 5% metallicnickel.
 5. A nickel-cadmium cell according to claim 4, wherein that theactive mass of the negative electrodes is free from metallic nickel. 6.In a reduced maintenance, vented nickel-cadmium cell including a set ofpositive electrodes, and a set of negative electrodes in which theactive mass is consolidated by a polymer, the improvement wherein:saidcell includes separators constituted by at least one felt ofpolypropylene and polyamide fibers, without an additional membrane, theweight of the felt being about 120 g/m², and said fibers having adiameter in the range 4 μm to 20 μm; the inter-electrode distance is inthe range 0.2 mm to 0.3 mm; the internal pressure is maintained in therange 0 bars to 0.7 bars relative pressure; and said cell contains freeelectrolyte above the electrodes; thereby presenting a chargingcharacteristic during constant voltage charging in the temperature range-30° C. to +60° C. that enables said charging to be self-limiting.
 7. Ina reduced maintenance, vented nickel-cadmium storage cell including aset of positive electrodes, and a set of negative electrodes in whichthe active mass is consolidated by a polymer or is electro-deposited,the improvement wherein:said cell includes separators constituted by atleast one felt made of a substance selected from a polyamide,polypropylene, polyethylene, individually or mixed together, and withoutan additional membrane; the inter-electrode distance is in the range 0.2mm to 0.3 mm; said felts have a weight lying in the range 90 g/m² to 120g/m² and are made of fibers, the majority of which have a diameter of 5μm; the internal pressure is maintained in the range 0 bars to 0.7 barsrelative pressure; and said cell contains free electrolyte above theelectrodes; thereby presenting a charging characteristic when chargingin the temperature range -30° C. to +60° C. enabling the end of chargingto be detected by measuring the rise in voltage.